Up to this time, several processes for preparing optically active abscisic acid have been known. These processes are classified into (i) a group of processes characterized by using an optically active raw material (see K. Mori, Tetrahedron Lett., 1973, 2635; M. Shibasaki, S. Terashima and S. Yamada, Chem. Pharm. Bull., 1976, 24, 315; and K. Kienzle, H. Mayer, R. E. Minder and H. Thommen, Helv. Chim. Acta, 1978, 61, 2616), (ii) a group of fermentation processes using a microorganism [see Japanese Patent Laid-Open No. 36393/1983 and the abstracts of the 30th Symposium on the Chemistry of Natural Products (Fukuoka, 1988), p.332 ] and (iii) a group of processes comprising preparing racemic abscisic acid which is not optically active and separating the racemic modification into optically active isomers by optical resolution (see R. S. Burden and H. F. Taylor, Pure & Appl. Chem., 1976, 47, 203).
However, the processes of the group (i) are disadvantageous in that the optically active raw material is generally difficultly available and the fermentation processes of the group (ii) are unfit for practical use owing to their low productivity. The optical resolution processes of the group (iii) have disadvantages due to optical resolution in that the operation is troublesome and the objective optically active substance can be obtained only in a low yield even by following a process using an optically inactive racemic modification of a compound represented by the general formula (VI) or (VII) which will be described below as an intermediate (see M. G. Constantino, P. Losco and E. E. Castellano, J. Org. Chem., 1989, 54, 681), which process is one of the most efficient processes among known processes for preparing a racemic modification.
Although optically active abscisic acid is demanded because of its higher activity as a plant hormone than that of racemic one (see R. S. Burden and H. F. Taylor, Pure & Appl. Chem., 1976, 47, 203), optically active abscisic acid is now much more expensive than racemic one.
On the other hand, not a few processes for preparing xanthoxin have also been known. For example, there have been known a process (i) characterized by using .beta.-ionone as a starting material (see R. S. Burder, G. W. Dawson and H. F. Taylor, Phytochem., 1972 11, 2295 and H. F. Taylor and R. S. Burder, J. Exp. Bot., 1973, 24, 873) and a process (ii) characterized by using isophorone as a starting material (see T. Oritani and K. Yamashita, Agr. Biol. Chem., 1973, 37, 1215). However, the processes (i) and (ii) are ones for preparing optically inactive racemic xanthoxin, being unimportant. Further, although a process (iii) for preparing optically active xanthoxin from an optically active 4-hydroxycyclocitral as a starting material has been also known (see F. Kienzle, H. Mayer, R. E. Minder and H. Thommen, Helv. Chim. Acta, 1978, 61, 2616), this process is also disadvantageous in that the optically active starting material is difficultly available.